The commercial production of fructose utilizes three distinct processes. The first is the partial hydrolysis of starch, with corn starch being most commonly used, but cassava, tapioca, and rice also being sources, where most of the starch molecules are converted to lower molecular weight polysaccharides, to afford a product called thinned starch or starch hydrolysate. The second process is the hydrolysis of thinned starch to glucose (saccharification) and is generally an enzymatic hydrolysis mediated by amyloglucosidase. The third process is the isomerization of glucose to fructose catalyzed by the enzyme glucose isomerase to afford a product commonly containing 42-45% fructose. Generally the three processes are performed largely independent of each other, i.e., as three distinct and uncoupled processes carried out in the usual course of fructose production. Contrastingly, the subject matter of this application is a method of making fructose from thinned starch where the second and third processes described above are closely linked and cooperate to afford results unattainable from uncoupled systems.
In recent years the glucose-to-fructose conversion has been catalyzed by immobilized glucose isomerase. The use of immobilized enzymes is attended by diverse advantages, including reuse of the enzyme, thereby permitting its more efficient utilization, and elimination of an enzyme removal stage, as would be necessary for reuse of a soluble enzyme. However, the use of immobilized amyloglucosidase (IMAG) in enzymatic hydrolysis of thinned starch has not achieved commercial success because of several disadvantages for which previously there were no solutions. One disadvantage of IMAG processing is that the maximum glucose attainable is about 93% when a high activity IMAG is utilized, whereas commercial processes attain 94-6% glucose levels using soluble AG. Another disadvantage of IMAG is the formation of relatively high levels of reversion product, especially isomaltose, a bitter principal which seriously impairs the quality of the resulting glucose solution when subsequently used as a feedstock for fructose production. Reversion products are formed by the action of AG on glucose. Their formation is dependent on glucose concentration and reaction time; high glucose concentrations and long reaction times favor increased formation of reversion products.
We have been able to solve the dual problems of an unacceptably low maximum glucose concentration and an unacceptably high reversion product concentration attending the use of an immobilized amyloglucosidase using as a process flow scheme the partial hydrolysis of thinned starch by AG, either soluble or immobilized, to a product containing from about 50% to about 85% glucose, isomerizing the latter product with IMGI, hydrolyzing the isomerized product with IMAG to a product containing 6% or less of disaccharides and higher oligosaccharides (DP.sub.2+), and isomerizing the saccharified stream with IMGI to the final desired fructose concentration. By conducting the initial hydrolysis to only partial conversion the accumulation of reversion products is reduced, both because the concentration of glucose is reduced and because a long reaction time is avoided. By conducting the second hydrolysis on a feedstock where glucose has been partially converted to fructose one avoids building up a high glucose concentration even where hydrolysis is conducted to the extent where not more than about 6% of disaccharides and higher oligosaccharides remain, and where the operational equivalent of at least 94% glucose has been attained, thereby tending to keep the accumulation of reversion product at reduced levels.
Thus the invention herein has the advantage of affording the operational equivalent of at least 94% glucose formation in the enzymatic hydrolysis of thinned starch using IMAG, a result heretofore unobtainable. A further advantage is that the equivalent of at least 94% glucose is formed by IMAG with a reduced level of isomaltose, a highly undesirable reversion product of glucose. Yet another advantage is that the scheme offers the potential of achieving superior results with an overall higher liquid hourly space velocity than a process using IMAG to maximum glucose formation followed by IMGI isomerization.